Method of rolling uranium



METHOD OF ROLLING Cyril S. Smith, Chicago, Ill., assignor to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Application July 18, 1949 Serial No. 105,453

4 Claims. (Cl. 8060) This invention relates to metal working and more particularly to methods for fabricating uranium metal. Specifically this invention pertains to a method for rolling uranium metal at relatively low temperatures and under nonoxidizing conditions.

Uranium metal is diflicult to work because it oxidizes readily in air and under certain conditions may become pyrophoric. Because of its high melting point, about 1150 C., and lack of malleability, uranium has been worked in the past at temperatures from 400 C. to 105 C. in a jacket of steel or other protective metal. These elevated temperatures cause markedly increased oxidation rates and necessitate precautionary measures to protect the worker from the effects of the radioactive oxide dust. Furthermore, the finished metal has many undesirable characteristics such as surface imperfections and inaccuracy of dimensions because uranium metal has a high coefiicient of expansion at advanced temperatures. Cold rolling of uranium metal, on the other hand, gives greatly improved surfaces over those obtained by hot rolling methods, but is disadvantageous in the greater time and labor involved and in the comparative brittleness of the worked material. Also, uranium metal cannot be reduced in thickness more than about 15 to 20 percent without extensive edge cracking, thus requiring annealing after very small reductions. These added annealing steps are time-consuming and uneconomical.

For many uses such as in nuclear experiments, power piles and the like, it is desirable for the uranium metal to be free of all impurities, including oxides. Furthermore, it is also desirable that the fabricated metal piece be free of surface imperfections and density variations throughout its volume.

It is a primary object of this invention to provide a simple, efficient method for fabricating uranium metal.

It is another object of this invention to provide a method for fabricating uranium metal which will produce a surface on the finished piece free of oxides and imperfections.

It is still another object of the present invention to provide a method for fabricating uranium metal to a uniform density.

It is a further object of the present invention to provide a method for fabricating uranium metal to accurate dimensions at moderate temperatures.

Still further objects and advantages of the present invention will beccome apparent to those skilled in the art from the description and examples which follow.

The objects of the present invention are achieved by fabricating uranium metal in the lower end of the alpha range at temperatures from 100 C. to 300 C., and preferably between 175 C. and 225 C. under non-oxidizing conditions. These temperatures are substantially below the recrystallization temperature of the metal, and thus cannot be considered to be in the range of hot working temperatures.

It has been found that if the metal is heated to approxi- I iC mately 200 C. it can be worked extensively without annealing, even after reductions as high as percent in thickness. The metal, in fact, surprisingly behaves much like the common malleable metals which are rolled or worked at room temperature. As stated hereinbefore, if uranium is rolled at room temperature, it cannot be reduced in thickness more than about 15 to 20 percent without extensive edge cracking. However, if the metal is heated to about 200 C. in accordance with the method of the present invention, such cracking is entirely eliminated, while all the advantages of cold rolling are preserved.

The heating of the metal for working can be performed conveniently in an oil bath, which has the advantages of providing rapid heating and of leaving a thin protective coating of oil on the specimen Withdrawn from the bath. Also, serious oxidation of the metal does not occur in the short time at the relatively low temperatures employed, and the danger of radioactive dust is greatly reduced.

The following example is given for the purpose of illustrating the method of the present invention by which uranium metal is fabricated into sheets.

Example I A block of unworked uranium metal approximately 2 inches square and one-half inch thick is placed in an oil bath. The bath is heated to a temperature of about 200 C. until the uranium block is uniformly heated throughout. At this time the uranium block is withdrawn from the bath and the excess oil permitted to drain, leaving a thin protective film of oil on the block. The blolck is then passed rapidly through a set of rolls adjusted to give a 15 pencent reduction in thickness in one pass. The rolling operation is repeated rapidly as many times as are necessary to reduce the block to a thickness of about oneeighth of an inch. This normally may be accomplished without an additional heating step, provided the rolls are maintained at a temperature of about 200 C.

The thin film of oil on the uranium piece is suflicient to protect the metal from the effects of oxidation throughout the entire rolling operation, although it is possible to supply a small additional amount of oil if desired. After the uranium metal sheet has been fabricated to the desired size and thickness the oil may be removed by dipping the sheet in any suitable solvent such as acetone, naphtha and the like. The cleaning operation is preferably performed in a dry box of the type well-known to those skilled in the art in order to minimize the danger to personnel from the uranium. The fabricated piece may then be stored or used as desired.

The following example illustrates another method of the present invention, in which the metal heating step is repeated between successive roll passes.

Example 11 A plate of uranium approximately one-fourth of an inch thick and of a width readily acceptable by the available rolling equipment is heated in an oil bath to a temperature of about 200 C. The plate is rolled as in the previous example but without heating the rolls and with reheating of the metal between each pass. When the plate has been reduced to a thickness of about one-tenth of an inch, the rolled metal is removed and cleaned as in the preceding example.

The above examples illustrate the simplicity and ease of operation of the method of the present invention. The method is not a hot rolling process because the uranium metal is fabricated at the temperatures below the recrystallization temperature of the metal. From the standpoint of structure of the metal, the method is essentially that of cold rolling. It should be pointed out, however,

that all of the disadvantages of cold rolling have been eliminated and that the resulting fabricated metal is free of brittleness, checking, edge cracking and other undesirable qualities, Furthermore, the fabricated pieces do not require annealing after each reduction in thickness, but may be reduced up to 90 percent by' either of the methods described in the foregoing examples without annealing. The finished pieces are malleable, clean, bright, and after cleaning are in condition for immediate use.

In the preceding examples reference has been made to the fabrication of uranium blocks and plates by rolling methods. Many other embodiments of the invention are, of course, possible. For example, other shapes and sizes of uranium metal may be fabricated by other metallurgic techniques such as pressing, swaging, forging, and other forming methods. The oil bath may consist of any mineral oil, silicone oil or non-reactive fluid that will not contaminate the uranium. Although the uranium may be worked at these low temperatures without excessive oxidation, it is preferred that non-oxidizing conditions be employed. Other methods utilizing, for example, an inert atmosphere of argon, helium or nitrogen, may be employed alone or along with the oil to prevent the oxidation of the uranium during the fabrication process. Furthermore, the uranium may be coated with silver or other inert metal during the fabricating steps, and that coating may be removed after the treatment.

While'the method does not ordinarily require annealing between the processing steps at the novel working temperature range, it is within the scope of the invention to provide such annealing steps. By working at low temperatures, particularly in the final stages of operation, it is easy to make the final shape in the proper dimensions. When using high temperatures, the large coefficient of expansion makes it difficult to insure exact and undistorted dimensions in the final cold piece.

The improved results obtained by the method of the present invention cannot be predicted on theory. Since disadvantages have been observed in prior fabricating methods which employed both room temperature and considerably elevated temperatures, the unexpectedly desirable results obtained by fabrication of the metal at moderately elevated temperatures, i.e., below hot working temperatures, is most striking.

While many embodiments of the invention are possible without departing from the spirit and scope of the invention, it is to be understood, however, that the invent-ion is not to be limited except as indicated in the appended claims.

What is claimed is:

1. In a method of working uranium metal the step which comprises rolling the metal at a temperature between 100 C. and 300 C.

2. The method of claim 1 in which the temperature is between about 175 C. and about 225 C.

3. The method of reducing the thickness of uranium metal in an amount up to percent which comprises heating the metal to 200 C. in an oil bath, withdrawing the uranium from the oil bath and permitting the oil to drain so that a thin protective coating of oil remains on the metal, rolling the heated uranium metal at a temperature of 200 C. to give about a 15 percent reduction in the thickness of the metal at each pass and repeating the rolling step until the metal is reduced to the desired size.

4. The method of reducing the thickness of uranium metal in an amount up to about 90 percent which comprises heating the metal in an oil bath to 200 C., rolling the heated uranium at a temperature between C. and 300 C. to give about a 15 percent reduction in the thickness of the metal at each pass and repeating the heating and rolling steps until the metal is reduced to the desired size.

References Cited in the file of this patent UNITED STATES PATENTS Hughes Aug. 11, 1914 Gero Aug. 24, 1926 OTHER REFERENCES 

1. IN A METHOD OF WORKING URANIUM METAL THE STEP WHICH COMPRISES ROLLING THE METAL AT A TEMPERATURE BETWEEN 100* C. AND 300* C. 